1. Field of the Invention
The present invention relates to a distortion removal apparatus for removing a harmonic distortion and an intermodulation distortion generated in a system such as a loudspeaker for reproducing, for example, an audio signal and performing high fidelity reproduction of an input signal, a method for determining a coefficient for the distortion removal apparatus. The present invention also relates to a processing speaker system, a multi-processor and an amplifier including the distortion moving apparatus
2. Description of the Related Art
As is well known, in order to reproduce music or announcement in a large space such as concert halls, outside theaters and stadiums, a large-size professional-use loudspeaker system for reproducing a large volume of sound is generally used. Specifically in order to reproduce sound in a high frequency range, a horn loudspeaker is used; and in order to reproduce sound in a low frequency range, a loudspeaker such as woofer or subwoofer is used.
A horn loudspeaker system includes a compression driver, which is an electro-acoustic transducer, and a horn into which sound wave output from the compression driver is input. The compression driver converts the input electric signal into sound wave and outputs the sound wave to the horn, and the horn radiates the sound wave into a large space. In the compression driver, the sound pressure radiated by a diaphragm for generating the sound wave by reducing the diameter of a part at which the compression driver is connected to the horn (a throat section of the horn) rather than the diameter of the diaphragm. Thus, reproduction of a large volume of sound is realized.
However, in such a compression driver, reproduction is distorted by high compression of the air and the distortion is added to the original sound wave. This is one of the causes of the distortion generated when a large volume of sound is reproduced by the horn loudspeaker system, which is quite serious.
In the case of reproduction of sound in a low frequency range in a large volume using a woofer, the diaphragm of the woofer is vibrated to a large amplitude. In this case, the reproduced sound accompanies a distortion due to the kinetic nonlinearity of the dumper or the edge supporting the diaphragm or the nonlinearity of the volume of the air eliminated by the edge.
The above-described distortions are nonlinear distortions referred to as the harmonic distortion and the intermodulation distortion. Such distortions, which are generated due to the above-described reasons when sound is generated in a large volume, deteriorate the sound quality.
A method for removing a distortion generated in a loudspeaker or the like which generates a nonlinear distortion (hereinafter, referred to as a xe2x80x9csystemxe2x80x9d) uses a nonlinear filter. One known nonlinear filter used for such general systems is a Volterra filter. The Volterra filter refers to connecting a one-dimensional filter, a two-dimensional filter, . . . an n-dimensional filter in parallel, performing a convolution operation of an input signal with respect to each filter, and then adding and outputting the results.
Such a method has a problem in that the amount of operation increases to an excessive degree as the number of dimensions increases from a two-dimensional filter to a three-dimensional filter and to an n-dimensional filter. For example, even in the case of removing only a two-order distortion from a loudspeaker as a system, a large-scale apparatus is required to perform a real-time two-dimensional convolution operation of an input audio signal.
The U.S. Pat. No. 4,709,391 to Kaizer at al. entitled xe2x80x9cArrangement for converting an electric signal into an acoustic signal or vice versa and a non-linear network for use in the arrangementxe2x80x9d (assigned to U.S. Philips Corporation) and the European Patent application No. 0 168 078 A1 filed in the names of the same inventors propose a nonlinear circuit for correcting a nonlinear distortion referred to as a two-order distortion or a three-order distortion of an electro-acoustic transducer. The method described in these documents is as follows. The nonlinearities of the magnetic flux density of the magnetic circuit of the loudspeaker, inductance of the voice coil, and spring constant of the damper are modelled by an equivalent circuit of an electric circuit to obtain respective constants. Then, the input and output characteristic of the loudspeaker is modelled by a series referred to as the Volterra series in which a usual transfer characteristic is connected in parallel to a nonlinear distortion characteristic. A nonlinear filter for providing an equivalent effect as that by the Volterra filter is connected as a distortion correction circuit to the stage prior to the loudspeaker. By the nonlinear filter, a distortion compensation signal for compensating for the distortion generated by the loudspeaker is added to the audio signal, and the resultant signal is sent to the loudspeaker.
By this method, a distortion removing effect which is equivalent to the effect obtained by two-dimensional and three-dimensional convolution operations is effected without performing such convolution operations.
However, this method requires a significant amount of trouble in order to measure constants for determining the nonlinear transfer characteristic of the loudspeaker, such as the magnetic flux density, inductance and spring constant. Moreover, when any of the measurement results includes a relatively large error and thus a satisfactory effect is not obtained by the above-described procedure, it is difficult to identify which of the constants includes an error. Thus, this method requires significant experience of the operator.
When this method is used for a system from which a nonlinear distortion is to be removed in order to, for example, remove a distortion from a dynamic electric loudspeaker, in the case where the factor of the distortion is clear, it is possible to design a distortion correction circuit. In the case where the distortion generation cannot be modelled by an equivalent circuit, such as in the case of a general system, it is impossible to design a distortion correction circuit.
The U.S. Pat. No. 5,438,625 to W. Klippel entitled xe2x80x9cArrangement to correct the linear and nonlinear transfer behavior or electro-acoustical transducersxe2x80x9d (assigned to JBL Incorporated) and the German Patent No. DE 41 11 884 C2 (granted to the same inventor) propose a nonlinear circuit for removing a nonlinear distortion in an electro-acoustic transducer and a method for automatically updating the coefficient for the circuit. According to the method described in these documents, as the method proposed by Kaizer et al., some factors of generation of a nonlinear distortion in the loudspeaker are formulated and a nonlinear circuit to be used as an inverse system is designed. This method is based on the concept of the Volterra filter, but constructs a distortion removing circuit not as a circuit for performing a two- or higher dimensional convolution operation but as a nonlinear circuit for providing the same effect as obtained by the convolution operation.
This method also requires formulation of some factors of the generation of distortion in the loudspeaker as a system in order to design a distortion removing circuit. Such a method is only usable for a system, the mechanism of generation of a distortion of which is clear. Moreover, it is necessary to measure the constants of the expressions after the formulation, which is quite complicated.
The two methods described in the above-described documents require formulation of the mechanism of generation of the distortion in a system in order to remove a distortion therefrom. Accordingly, when the factors of the distortion are not clearly known or when formulation cannot be performed with high precision, these methods cannot be used.
According to one aspect of the invention, a distortion removal apparatus connected between a signal source and a signal input section of a distortion-generating system, for processing a signal output from the signal source so as to compensate for a distortion component generated in the system is provided. The distortion removal apparatus includes a frame division section for dividing a signal output from the signal source into data streams having a length of N while causing the data streams to partially overlap with one another; a Fourier transform section for performing Fourier transform of the data streams obtained by the frame division section in a time domain into a signal in a frequency domain; a memory section for storing N samples of first coefficients and Nxc3x97N samples of second coefficients in the frequency domain; an operation section for removing a distortion component from the output signal from the Fourier transform section by performing an operation based on the first coefficients and the output signal from the Fourier transform section and an operation based on the second coefficients and the output signal from the Fourier transform section; an inverse Fourier transform section for performing inverse Fourier transform of the output signal from the operation section into a signal in the time domain; and a frame synthesis section for sequentially connecting parts of the output signal from the inverse Fourier transform section.
According to another aspect of the invention, a distortion removal apparatus connected between a signal source and a signal input section of a distortion-generating system, for processing a signal output from the signal source so as to compensate for a distortion component generated in the system is provided. The distortion removal apparatus includes a frame division section for dividing a signal output from the signal source into data streams having a length of N while causing the data streams to partially overlap with one another; a Fourier transform section for performing Fourier transform of the data streams obtained by the frame division section in a time domain into a signal in a frequency domain; a first memory section for storing N samples of first coefficients in a frequency domain; and a first multiplier for performing a multiplication of a first term of the right side of expression (1) using the first coefficients and the output signal from the Fourier transform section,                                                                         W                ⁡                                  (                  m                  )                                            =                              xe2x80x83                            ⁢                                                                    G1                    ⁡                                          (                      m                      )                                                        ⁢                                      X                    ⁡                                          (                      m                      )                                                                      +                                                                                                        xe2x80x83                            ⁢                                                ∑                                                            m                      =                                              m1                        +                        m2                                                              ⁢                                          
                                        ⁢                    or                    ⁢                                          
                                        ⁢                                          "LeftBracketingBar"                                              m1                        -                        m2                                            "RightBracketingBar"                                                                      ⁢                                                      G2                    ⁡                                          (                                              m1                        ,                        m2                                            )                                                        ⁢                                      X                    ⁡                                          (                      m1                      )                                                        ⁢                                      X                    ⁡                                          (                      m2                      )                                                                                                                              expression        ⁢                  xe2x80x83                ⁢                  (          1          )                    
where m, m1 and m2 each represent an integer representing the number of discrete points on a frequency axis, W(m) represents a component of an m""th frequency point of the output signal from the distortion removal apparatus in a frequency domain: G1(m) represents the first coefficient, X(m) represents an m component of the signal in the frequency domain obtained by the Fourier transform of an input signal after the input signal is made discrete, G2(m1, m2) represents a second coefficient, X(m1) represents an m1 component of the signal in the frequency domain obtained by the Fourier transform of the input signal after the input signal is made discrete, and X(m2) represents an m2 component of the signal in the frequency domain obtained by the Fourier transform of the input signal after the input signal is made discrete. The distortion removal apparatus further includes a second memory section for storing Nxc3x97N samples of second coefficients in the frequency domain; a multiplier and adder for performing a multiplication and an addition of the second term of the right side of expression (1) using the second coefficients and the output signal from the Fourier transform section; an adder for adding the output signal from the first multiplier and the output signal from the multiplier and adder; an inverse Fourier transform section for performing inverse Fourier transform of the output signal from the adder into a signal in the time domain; and a frame synthesis section for sequentially connecting parts of the output signal from the inverse Fourier transform section and outputting the resultant signal.
According to still another aspect of the invention, a distortion removal apparatus connected between a signal source and a signal input section of a distortion-generating system, for processing a signal output from the signal source so as to compensate for a distortion component generated in the system is provided. The distortion removal apparatus includes a frame division section for dividing a signal output from the signal source into data streams having a length of N while causing the data streams to partially overlap with one another; a Fourier transform section for performing Fourier transform of the data streams obtained by the frame division section in a time domain into a signal in a frequency domain; a third memory section for storing a tap in a frequency domain obtained by Fourier transform of an impulse response from an N-tapped delay section having a delay amount which is substantially equivalent to a group delay of a first-order impulse response from the system from which a distortion is to be removed; a first multiplier for reading the tap stored in the third memory section and performing a multiplication of the tap by the output signal from the Fourier transform section; a first memory section for storing N samples of first coefficients in the frequency domain; and a second multiplier for performing a multiplication of the first term of the right side of expression (1) using the first coefficients and the output signal from the first multiplier,                                                                         W                ⁡                                  (                  m                  )                                            =                              xe2x80x83                            ⁢                                                                    G1                    ⁡                                          (                      m                      )                                                        ⁢                                      X                    ⁡                                          (                      m                      )                                                                      +                                                                                                        xe2x80x83                            ⁢                                                ∑                                                            m                      =                                              m1                        +                        m2                                                              ⁢                                          
                                        ⁢                    or                    ⁢                                          
                                        ⁢                                          "LeftBracketingBar"                                              m1                        -                        m2                                            "RightBracketingBar"                                                                      ⁢                                                      G2                    ⁡                                          (                                              m1                        ,                        m2                                            )                                                        ⁢                                      X                    ⁡                                          (                      m1                      )                                                        ⁢                                      X                    ⁡                                          (                      m2                      )                                                                                                                              expression        ⁢                  xe2x80x83                ⁢                  (          1          )                    
where m, m1 and m2 each represent an integer representing the number of discrete points on a frequency axis, W(m) represents a component of an m""th frequency point of the output signal from the distortion removal apparatus in a frequency domain; G1(m) represents the first coefficient, X(m) represents an m component of the signal in the frequency domain obtained by the Fourier transform of an input signal after the input signal is made discrete, G2(m1, m2) represents a second coefficient, X(m1) represents an m1 component of the signal in the frequency domain obtained by the Fourier transform of the input signal after the input signal is made discrete, and X(m2) represents an m2 component of the signal in the frequency domain obtained by the Fourier transform of the input signal after the input signal is made discrete. The distortion removal apparatus further includes a second memory section for storing Nxc3x97N samples of second coefficients in the frequency domain; a multiplier and adder for performing a multiplication and an addition of the second term of the right side of expression (1) using the second coefficients and the output signal from the first multiplier; an adder for adding the output signal from the second multiplier and the output signal from the multiplier and adder; an inverse Fourier transform section for performing inverse Fourier transform of the output signal from the adder into a signal in the time domain; and a frame synthesis section for sequentially connecting parts of the output signal from the inverse Fourier transform section and outputting the resultant signal
According to still another aspect of the invention, a method for determining a coefficient for a distortion removal apparatus according to one of the above-described apparatuses will be described. A first coefficient G1(m) which is a numerical value stream having N numerical values is obtained by a process comprising the steps of measuring a transfer function of a first-order subsystem of the system as a stream of N numerical values having a point number of N2 in the frequency domain, where N2xe2x89xa6N1 less than N; performing inverse Fourier transform of the numerical value stream in the frequency domain into a numerical value stream having a point number of N2 in the time domain; adding Nxe2x88x92N2 pieces of zero data to a trailing end of the numerical value stream in the time domain; performing Fourier transform of the numerical value stream having a length of N including the zero data into a numerical value stream having a point number of N in the frequency domain; and storing the resultant numerical value stream in the frequency domain in a memory section as a first coefficient. Nxc3x97N samples of second coefficients G2(m1, m2) are obtained by a process comprising the steps of measuring a transfer function of a second-order subsystem which generates a distortion of the system as a numerical value arrangement having a point number of N3xc3x97N3 in the frequency domain, where N3xe2x89xa6N1 less than N; performing two-dimensional inverse Fourier transform of the numerical value arrangement in the frequency domain into a numerical value arrangement having a point number of N3xc3x97N3 in the time domain; adding Nxe2x88x92N3 pieces of zero data in a row direction and a column direction to a trailing end of the numerical value arrangement in the time domain to form a numerical value arrangement of Nxc3x97N; performing two-dimensional Fourier transform of the numerical value arrangement having a point number of Nxc3x97N including the zero data into a numerical value arrangement having a point number of Nxc3x97N in the frequency domain; and storing the resultant numerical value arrangement having a point number of Nxc3x97N in the frequency domain in a memory section as second coefficients.
According to still another aspect of the invention, a distortion removal apparatus connected between a signal source and a signal input section of a distortion-generating system, for adding a characteristic of a low-pass filter to a signal output from the signal source and also processing the signal output from the signal source so as to compensate for a distortion component generated in the system is provided. The distortion removal apparatus includes an A/D converter for converting an analog signal output from the signal source into a digital signal; a first filter for performing a one-dimensional convolution operation of the output signal from the A/D converter; a second filter for performing a two-dimensional convolution operation of the output signal from the A/D converter; an adder for adding the output signal from the first filter and the output signal from the second filter; and a D/A converter for converting the output signal from the adder into an analog signal. Where m is an integer representing a point number on a frequency axis, m1 and m2 are each an integer representing a point number on the frequency axis and fulfill the relationship of m=m1+m2 or m=|m1xe2x88x92m2|, H1(m) is a value representing the transfer characteristic of a first-order subsystem of the system in a frequency domain, and H2(m1, m2) is a value representing the transfer characteristic of a second-order harmonic distortion and intermodulation distortion of a second-order subsystem of the system in a two-dimensional frequency domain; the transfer characteristic G1L(m) of the first filter is G1(m)L(m) obtained by multiplying a characteristic L(m) of the low-pass filter by an arbitrarily determinable first-order transfer characteristic G1(m), and the transfer characteristic G2L(m1, m2) of the second filter is represented by expression (16),                               G2L          ⁡                      (                          m1              ,              m2                        )                          =                              -                                          H2                ⁡                                  (                                      m1                    ,                    m2                                    )                                                            H1                ⁡                                  (                  m                  )                                                              ⁢                      G1L            ⁡                          (              m1              )                                ⁢                                    G1L              ⁡                              (                m2                )                                      .                                              expression        ⁢                  xe2x80x83                ⁢                  (          16          )                    
According to still another aspect of the invention, a distortion removal apparatus connected between a signal source and a signal input section of a distortion-generating system, for adding a characteristic of a band-pass filter to a signal output from the signal source and also processing the signal output from the signal source so as to compensate for a distortion component generated in the system is provided. The distortion removal apparatus includes an A/D converter for converting an analog signal output from the signal source into a digital signal; a first filter for performing a one-dimensional convolution operation of the output signal from the A/D converter; a second filter for performing a two-dimensional convolution operation of the output signal from the A/D converter; an adder for adding the output signal from the first filter and the output signal from the second filter; and a D/A converter for converting the output signal from the adder into an analog signal. Where m is an integer representing a point number on a frequency axis, m1 and m2 are each an integer representing a point number on the frequency axis and fulfill the relationship of m=m1+m2 or m=|m1xe2x88x92m2|, H1(m) is a value representing the transfer characteristic of a first-order subsystem of the system in a frequency domain, and H2(m1, m2) is a value representing the transfer characteristic of a second-order harmonic distortion and intermodulation distortion of a second-order subsystem of the system in a two-dimensional frequency domain; the transfer characteristic G1B(m) of the first filter is G1(m)B(m) obtained by multiplying a characteristic B(m) of the band-pass filter by an arbitrarily determinable first-order transfer characteristic G1(m), and the transfer characteristic G2(m1, m2) of the second filter is represented by expression (24),                               G2B          ⁡                      (                          m1              ,              m2                        )                          =                              -                                          H2                ⁡                                  (                                      m1                    ,                    m2                                    )                                                            H1                ⁡                                  (                  m                  )                                                              ⁢                      G1B            ⁡                          (              m1              )                                ⁢                                    G1B              ⁡                              (                m2                )                                      .                                              expression        ⁢                  xe2x80x83                ⁢                  (          24          )                    
According to still another aspect of the invention, a distortion removal apparatus connected between a signal source and a signal input section of a distortion-generating system, for adding a characteristic of a high-pass filter to a signal output from the signal source and also processing the signal output from the signal source so as to compensate for a distortion component generated in the system is provided. The distortion removal apparatus includes an A/D converter for converting an analog signal output from the signal source into a digital signal; a first filter for performing a one-dimensional convolution operation of the output signal from the A/D converter; a second filter for performing a two-dimensional convolution operation of the output signal from the A/D converter; an adder for adding the output signal from the first filter and the output signal from the second filter; and a D/A converter for converting the output signal from the adder into an analog signal. Where m is an integer representing a point number on a frequency axis, m1 and m2 are each an integer representing a point number on the frequency axis and fulfill the relationship of m=m1+m2 or m=|1xe2x88x92m2|, H1(m) is a value representing the transfer characteristic of a first-order subsystem of the system in a frequency domain, and H2(m1, m2) is a value representing the transfer characteristic of second-order harmonic distortion and intermodulation distortion of a second-order subsystem of the system in a two-dimensional frequency domain: the transfer characteristic G1F(m) of the first filter is G1(m)F(m) obtained by multiplying a characteristic F(m) of the high-pass filter by an arbitrarily determinable first-order transfer characteristic G1(m), and the transfer characteristic G2F(m1, m2) of the second filter is represented by expression (32),                               G2F          ⁡                      (                          m1              ,              m2                        )                          =                              -                                          H2                ⁡                                  (                                      m1                    ,                    m2                                    )                                                            H1                ⁡                                  (                  m                  )                                                              ⁢                      G1F            ⁡                          (              m1              )                                ⁢                                    G1F              ⁡                              (                m2                )                                      .                                              expression        ⁢                  xe2x80x83                ⁢                  (          32          )                    
According to still another aspect of the invention, a distortion removal apparatus connected between a signal source and a signal input section of a distortion-generating system, for processing a signal output from the signal source so as to compensate for a distortion component generated in the system is provided. The distortion removal apparatus includes a first filter for performing a first-order convolution operation of the output signal from the signal source; a second filter for performing a second-order convolution operation of the output signal from the signal source; a low-pass filter to which the output signal from the second filter is input; and an adder for adding the output signal from the first filter and the output signal from the low-pass filter. The cut-off frequency of the low-pass filter is less than the upper limit of a reproduction frequency band of the system.
According to still another aspect of the invention, a distortion removal apparatus connected between a signal source and a signal input section of a distortion-generating system, for processing a signal output from the signal source so as to compensate for a distortion component generated in the system is provided. The distortion removal apparatus includes a first filter for performing a first-order convolution operation of the output signal from the signal source; a second filter for performing a second-order convolution operation of the output signal from the signal source; and an adder for adding the output signal from the first filter and the output signal from the low-pass filter. Where X(m) is a value representing an input signal from the signal source in a frequency domain, m is an integer representing a point of a frequency axis, m1 and m2 are each a value fulfilling the relationship of m=m1+m2 or m=|m1xe2x88x92m2|, H1(m) is a value representing the transfer characteristic of a first-order subsystem of the system in a frequency domain, and H2(m1, m2) is a value representing the transfer characteristic of second-order harmonic distortion and intermodulation distortion of a second-order subsystem of the system in a two-dimensional frequency domain, G1(m) is a value representing the transfer characteristic of the first filter in the frequency domain, G2(m1, m2) is a value representing the transfer characteristic of the second filter in the frequency domain, and mc is an integer corresponding to the cut-off frequency on the high frequency range side of the transfer characteristic of the first-order subsystem w of the system; the second filter is a two-dimensional digital filter having a tap length of Nxc3x97N, provides the transfer characteristic G2(m1, m2) with respect to the input signal X(m) and outputs a result of a convolution operation of the input signal X(m) performed in an area represented by expression (36) while blocking the area other than the area represented by expression (36),                               G2          ⁡                      (                          m1              ,              m2                        )                          =                              -                                          H2                ⁡                                  (                                      m1                    ,                    m2                                    )                                                            H1                ⁡                                  (                  m                  )                                                              ⁢                      G1            ⁡                          (              m1              )                                ⁢                      G1            ⁡                          (              m2              )                                                          expression        ⁢                  xe2x80x83                ⁢                  (          9          )                    xe2x80x83m1+m2xe2x89xa6mc, m1xe2x89xa70, m2xe2x89xa70
Nxe2x88x92mcxe2x89xa6m1+m2xe2x89xa6N+mc, 0xe2x89xa6m1 less than N, 0xe2x89xa6m2 less than N
m1+m2xe2x89xa72Nxe2x88x92mc, m1 less than N, m2 less than Nxe2x80x83xe2x80x83expression (36).
According to still another aspect of the invention, a distortion removal apparatus connected between a signal source and a signal input section of a distortion-generating system, for processing a signal output from the signal source so as to compensate for a distortion component generated in the system is provided The distortion removal apparatus includes a first filter for performing a first-order convolution operation of the output signal from the signal source; a second filter for performing a second-order convolution operation of the output signal from the signal source; a band-pass filter to which the output signal from the second filter is input; and an adder for adding the output signal from the first filter and the output signal from the low-pass filter. The cut-off frequency of the band-pass filter on the low frequency range side is equal to or more than the upper limit of a reproduction frequency band of the system, and the cut-off frequency of the band-pass filter on the high frequency range side is equal to or less than the upper limit of the reproduction frequency band of the system.
According to still another aspect of the invention, a distortion removal apparatus connected between a signal source and a signal input section of a distortion-generating system, for processing a signal output from the signal source so as to compensate for a distortion component generated in the system is provided. The distortion removal apparatus includes a first filter for performing a first-order convolution operation of the output signal from the signal source; a second filter for performing a second-order convolution operation of the output signal from the signal source; and an adder for adding the output signal from the first filter and the output signal from the band-pass filter. Where X(m) is a value representing an input signal from the signal source in a frequency domain, m is an integer representing a point of a frequency axis, m1 and m2 are each a value fulfilling the relationship of m=m1+m2 or m=|m1xe2x88x92m2|, H1(m) is a value representing the transfer characteristic of a first-order subsystem of the system in a frequency domain, and H2(m1, m2) is a value representing the transfer characteristic of second-order harmonic distortion and intermodulation distortion of a second-order subsystem of the system in a two-dimensional frequency domain, G1(m) is a value representing the transfer characteristic of the first filter in the frequency domain, G2(m1, m2) is a value representing the transfer characteristic of the second filter in the frequency domain, and mc is an integer corresponding to the cut-off frequency on the high frequency range side of the transfer characteristic of the first-order subsystem of the system; the second filter is a two-dimensional digital filter having a tap length of Nxc3x97N, provides the transfer characteristic G2(m1, m2) represented by expression (9) with respect to the input signal X(m), and outputs a result of a convolution operation of the input signal X(m) performed in an area represented by expression (39) while blocking the area other than the area represented by expression (39),                               G2          ⁡                      (                          m1              ,              m2                        )                          =                              -                                          H2                ⁡                                  (                                      m1                    ,                    m2                                    )                                                            H1                ⁡                                  (                  m                  )                                                              ⁢                      G1            ⁡                          (              m1              )                                ⁢                      G1            ⁡                          (              m2              )                                                          expression        ⁢                  xe2x80x83                ⁢                  (          9          )                    xe2x80x83mclxe2x89xa6m1+m2xe2x89xa6mch, 0xe2x89xa6m1, 0xe2x89xa6m2
Nxe2x88x92mchxe2x89xa6m1+m2xe2x89xa6Nxe2x88x92mcl, 0xe2x89xa6m1, 0xe2x89xa6m2
N+mclxe2x89xa6m1+m2xe2x89xa6N+mch, m1 less than N, m2 less than N
2Nxe2x88x92mchxe2x89xa6m1+m2xe2x89xa62Nxe2x88x92mcl, m1 less than N, m2 less than Nxe2x80x83xe2x80x83expression (39).
According to still another aspect of the invention, a distortion removal apparatus connected between a signal source and a signal input section of a distortion-generating system, for processing a signal output from the signal source so as to compensate for a distortion component generated in the system is provided. The distortion removal apparatus includes a first filter for performing a first-order convolution operation of the output signal from the signal source; a second filter for performing a second-order convolution operation of the output signal from the signal source: a high-pass filter to which the output signal from the second filter is input; and an adder for adding the output signal from the first filter and the output signal from the high-pass filter. The cut-off frequency of the high-pass filter is equal to or more than the upper limit of a reproduction frequency band of the system.
According to still another aspect of the invention, a distortion removal apparatus connected between a signal source and a signal input section of a distortion-generating system, for processing a signal output from the signal source so as to compensate for a distortion component generated in the system is provided. The distortion removal apparatus includes a first filter for performing a first-order convolution operation of the output signal from the signal source; a second filter for performing a second-order convolution operation of the output signal from the signal source; and an adder for adding the output signal from the first filter and the output signal from the second filter. Where X(m) is a value representing an input signal from the signal source in a frequency domain, m is an integer representing a point of a frequency axis, m1 and m2 are each a value fulfilling the relationship of m=m1+m2 or m=|m1xe2x88x92m2|, H1(m) is a value representing the transfer characteristic of a first-order subsystem of the system in a frequency domain, and H2(m1, m2) is a value representing the transfer characteristic of second-order harmonic distortion and intermodulation distortion of a second-order subsystem of the system in a two-dimensional frequency domain, G1(m) is a value representing the transfer characteristic of the first filter in the frequency domain, G2(m1, m2) is a value representing the transfer characteristic of the second filter in the frequency domain, and mc is an integer corresponding to the cut-off frequency on the high frequency range side of the transfer characteristic of the first-order subsystem of the system; the second filter is a two-dimensional digital filter having a tap length of Nxc3x97N, provides the transfer characteristic G2(m1, m2) represented by expression (9) with respect to the input signal X(m), and outputs a result of a convolution operation of the input signal X(m) performed in an area represented by expression (42) while blocking the area other than the area represented by expression (42),                               G2          ⁡                      (                          m1              ,              m2                        )                          =                              -                                          H2                ⁡                                  (                                      m1                    ,                    m2                                    )                                                            H1                ⁡                                  (                  m                  )                                                              ⁢                      G1            ⁡                          (              m1              )                                ⁢                      G1            ⁡                          (              m2              )                                                          expression        ⁢                  xe2x80x83                ⁢                  (          9          )                    xe2x80x83mcxe2x89xa6m1+m2xe2x89xa6Nxe2x88x92mc, 0xe2x89xa6m1, 0xe2x89xa6m2
N+mcxe2x89xa6m1+m2xe2x89xa62Nxe2x88x92mc, m1 less than N, m2 less than Nxe2x80x83xe2x80x83(42).
According to another still aspect of the invention, a distortion removal apparatus connected between a signal source and a signal input section of a processor, which is provided on a stage before a distortion-generating system, for processing a signal output from the signal source so as to compensate for a distortion component generated in the system is provided. The w distortion removal apparatus includes a first filter for performing a first-order convolution operation of the output signal from the signal source; a second filter for performing a second-order convolution operation of the output signal from the signal source; and an adder for adding the output signal from the first filter and the output signal from the second filter. Where X(m) is a value representing an input signal from the signal source in a frequency domain, m is an integer representing a point of a frequency axis, m1 and m2 are each a value fulfilling the relationship of m=m1+m2 or m=|m1xe2x88x92m2|, H1(m) is a value representing the transfer characteristic of a first-order subsystem of the system in a frequency domain, and H2(m1, m2) is a value representing the transfer characteristic of second-order harmonic distortion and intermodulation distortion of a second-order subsystem of the system in a two-dimensional frequency domain, E(m) is a value representing a transfer characteristic of the processor in the frequency domain, G1(m) is a value representing the transfer characteristic of the first filter in the frequency domain, and G2(m1, m2) is a value representing the transfer characteristic of the second filter in the frequency domain; the second filter is a two-dimensional digital filter having a tap length of Nxc3x97N, and outputs a result of a convolution operation of the input signal X(m) performed using the two-dimensional transfer characteristic G2(m1, m2) represented by expression (48),                               G2          ⁡                      (                          m1              ,              m2                        )                          =                              -                                                            H2                  ⁡                                      (                                          m1                      ,                      m2                                        )                                                  ⁢                                  E                  ⁡                                      (                    m1                    )                                                  ⁢                                  E                  ⁡                                      (                    m2                    )                                                                                                H1                  ⁡                                      (                    m                    )                                                  ⁢                                  E                  ⁡                                      (                    m                    )                                                                                ⁢                      G1            ⁡                          (              m1              )                                ⁢                                    G1              ⁡                              (                m2                )                                      .                                              expression        ⁢                  xe2x80x83                ⁢                  (          48          )                    
According to still another aspect of the invention, a distortion removal apparatus connected between a signal source and a signal input section of a processor, which is provided on a stage before a distortion-generating system, for processing a signal output from the signal source so as to compensate for a distortion component generated in the processor is provided. The distortion removal apparatus includes a first filter for performing a first-order convolution operation of the output signal from the signal source; a second filter for performing a second-order convolution operation of the output signal from the signal source; and an adder for adding the output signal from the first filter and the output signal from the second filter. Where X(m) is a value representing an input signal from the signal source in a frequency domain, m is an integer representing a point of a frequency axis, m1 and m2 are each a value fulfilling the relationship of m=m1+m2 or m=|m1xe2x88x92|m2|, H1(m) is a value representing the transfer characteristic of a first-order subsystem of the system in a frequency domain, and H2(m1, m2) is a value representing the transfer characteristic of second-order harmonic distortion and intermodulation distortion of a second-order subsystem of the system in a two-dimensional frequency domain, A1(m) is a value representing a transfer characteristic of a first-order subsystem of the processor in the frequency domain, A2(m1, m2) is a value representing a transfer characteristic of a second-order subsystem of the processor in the frequency domain, G1(m) is a value representing the transfer characteristic of the first filter in the frequency domain, and G2(m1, m2) is a value representing the transfer characteristic of the second filter in the frequency domain; the second filter is a two-dimensional digital filter having a tap length of Nxc3x97N, and outputs a result of a convolution operation of the input signal X(m) performed using the two-dimensional transfer characteristic G2(m1, m2) represented by expression (52),                               G2          ⁡                      (                          m1              ,              m2                        )                          =                              -                                                                                                                                                        H1                          ⁡                                                      (                            m1                            )                                                                          ⁢                                                  A2                          ⁡                                                      (                                                          m1                              ,                              m2                                                        )                                                                                              +                                                                                                                                                          H2                        ⁡                                                  (                                                      m1                            ,                            m2                                                    )                                                                    ⁢                                              A1                        ⁡                                                  (                          m1                          )                                                                    ⁢                                              A1                        ⁡                                                  (                          m2                          )                                                                                                                                                                  H1                  ⁡                                      (                    m                    )                                                  ⁢                                  A1                  ⁡                                      (                    m                    )                                                                                ⁢                      G1            ⁡                          (              m1              )                                ⁢                                    G1              ⁡                              (                m2                )                                      .                                              expression        ⁢                  xe2x80x83                ⁢                  (          52          )                    
According to still another aspect of the invention, a multi-processor including any one of the above-described distortion removal apparatuses is provided. The distortion-generating system is a loudspeaker for reproducing an audio signal, and the system is provided between the signal source and an amplifier for driving the loudspeaker.
According to still another aspect of the invention, an amplifier including any one of the above-described distortion removal apparatuses is provided. The distortion-generating system is a loudspeaker for reproducing an audio signal, and the amplifier drives the loudspeaker.
According to still another aspect of the invention, a processing speaker system including any one of the above-described distortion removal apparatuses is provided. The distortion-generating system is a loudspeaker for reproducing an audio signal.
As described above, according to the present invention, in order to remove a distortion, the characteristic of a distortion generated in a loudspeaker is measured in advance, and a coefficient for generating a distortion compensation signal is obtained based on the measurement result. This coefficient is stored in a distortion removal apparatus. The distortion removal apparatus generates a distortion compensation signal using the coefficient upon receipt of an audio signal, and adds the distortion compensation signal to the original audio signal. The combined signal is output to the loudspeaker via a power amplifier. The distortion generated in the loudspeaker is cancelled by the distortion compensation signal, and only the audio signal output from the signal source is radiated from the loudspeaker. As a result, the loudspeaker reproduces the audio signal with high fidelity without generating a distortion.
According to the present invention, the trouble for analyzing in detail the factor of generation of a distortion in a system, for example, a loudspeaker can be saved.
According to the present invention, a distortion compensation signal is generated real-time with respect to an arbitrary input audio signal.
According to the present invention, the function of a band restricting filter of allowing only a component of the input signal in a certain frequency range to pass for output is provided.
According to the present invention, the amount of operation performed by a second-order filter can be reduced in a distortion removal apparatus having the function of a band restricting filter.
According to the present invention, even when a graphic equalizer is provided on the stage after the distortion removal apparatus, the distortion in the loudspeaker can be removed.
According to the present invention, when the power amplifier for driving the loudspeaker generates a nonlinear distortion, the distortion in the amplifier can be also removed.
Thus, the invention described herein makes possible the advantages of providing (1) a practical distortion removal apparatus, disposed at a stage prior to a distortion-generating system such as a loudspeaker, for generating a signal for compensating for the distortion generated in the system with high precision, (2) a method for determining a coefficient usable in the distortion removal apparatus, (3) a processing speaker system, multi-processor and amplifier including the distortion removal apparatus, and (4) a distortion removal apparatus for reducing the amount of operation performed for signal processing therein.
These and other advantages of the present invention will become apparent to those skilled in the art upon reading and understanding the following detailed description with reference to the accompanying figures.